Origin and domestication

Unlike other crops, there is no archaeological evidence to indicate how long S. edule has been cultivated. Its fleshy fruit, which has a single seed with a smooth testa, does not allow it to be preserved and, as far as is known, no pollen grains or other structure of this species have been identified on archaeological sites.

Chroniclers from the time of the conquest record that, in Mexico at least, the chayote has been cultivated since pre-Columbian times. As regards linguistic references. the common names of native origin are concentrated mainly in Mexico and Central America. Exploration records concur in the finding that the widest variation of S. edule under cultivation is found between southern Mexico and Guatemala. The geographical distribution of the wild relatives of S. edule also testifies to the Mesoamerican origin of this crop.

The closest relatives to S. edule are:

the so-called wild forms of S. edule, the taxonomic positions of which are unresolved since they are distributed in an apparently natural way in the Mexican states of Veracruz, Puebla, Hidalgo, Oaxaca and Chiapas;

S. compositum, a species restricted to southern Mexico (Chiapas) and Guatemala;

S. hintonii, a species endemic to Mexico, until recently considered to be extinct and which grows in the states of Mexico and Guerrero and possibly in Jalisco;

a new species of the Sechium selection which grows in the north of the state of Oaxaca.

From the foregoing it has been possible to corroborate the fact that S. edule is a species which was undoubtedly domesticated within the cultural area of Mesoamerica, and specifically in the region lying between southern Mexico and Guatemala.

Chayote cultivation is widely distributed in Mesoamerica. It was introduced into the Antilles and South America between the eighteenth and nineteenth centuries. The first botanical description mentioning the name Sechium was in fact done in 1756 by P. Brown who referred to plants grown in Jamaica. During this same period, the chayote was introduced into Europe whence it was taken to Africa, Asia and Australia, while its introduction into the United States dates from the late nineteenth century.

Uses and nutritional value

The chayote is used mainly for human consumption. The fruit, stems and young leaves as well as the tuberized portions of the roots are eaten as a vegetable, both alone and plain boiled, and as an ingredient of numerous stews. Because of its softness, the fruit has been used for children's food. juices, sauces and pasta dishes. In Mexico, an attempt has been made to increase the life of the fruit by drying it. The results have been positive and have enabled jams and other sweets to be prepared while also producing dried fruit which can be used as a vegetable after a certain time. Because of their flexibility and strength, the stems have been used in the craft manufacture of baskets and hats. In India, the fruit and roots are not only used as human food but also as fodder.

The edible parts of S. edule have a lower fibre, protein and vitamin content than other plants. However, the calorie and carbohydrate content is high, chiefly in the case of the young stems, root and seed, while the micronutrients and macronutrients supplied by the fruit are adequate. The fruit and particularly the seeds are rich in amino acids such as aspartic acid, glutamic acid, alanine, arginine, cysteine, phenylalanine, glycine, histidine, isoleucine, leucine, methionine (only in the fruit), proline, serine, tyrosine, threonine and valine.

The chayote also has medicinal uses; infusions of the leaves are used to dissolve kidney stones and to assist in the treatment of arteriosclerosis and hypertension; infusions of the fruit are used to alleviate urine retention. The cardiovascular properties of the infusions of leaves have been tested in modern studies, while their great effectiveness in curing kidney diseases has been known since colonial times on the Yucatán peninsula, where these ailments are very common.

Botanical description

S. edule is a perennial, monoecious climber, with thickened roots and slender, branching stems up to 10 m long. Its leaves are on sulcate petioles of 8 to 15 cm in length, they are ovate-cordate to suborbicular, measure 8 to 18 x 9 to 22 cm, are slightly lobate (with three to five angular lobes) and have minutely denticulate margins and three to five divided tendrils.

The flowers are unisexual, normally pentamerous, coaxillary and with ten nectaries in the form of a pore at the base of the calyx. The staminate flowers grow in axillary racemose inflorescences that are 10 to 30 cm long, and the groups of flowers are distributed at intervals along the rachis. The calyx is patteliform and 5 mm wide, the sepals triangular and 3 to 6 mm long, the petals triangular, greenish to greenish-white and measure 4 to 8 x 2 to 3 mm. There are five stamens, and the filaments are fused almost along their total length, forming a thickened column which separates at the apex into three or five short branches. The pistillate flowers are normally on the same axilla as the staminate flowers; they are usually solitary but are occasionally in pairs; the ovary is globose, ovoid or piriform, glabrous, inerm and unilocular; the perianth is as in the staminate flowers but has slightly different dimensions; the styles are fused in a slender column and the nectaries are generally less evident than in the staminate flowers. The fruit is solitary or rarely occurs in pairs; it is viviparous, fleshy and sometimes longitudinally sulcate or crestate; it is of very different shapes and sizes, indumentum, number and type of spines; it is white and yellowish, or pale green to dark green with a pale green to whitish flesh that is bitter in the wild plants and not bitter in the cultivated ones. The seed is ovoid and compressed with a soft and smooth testa.

Figure 6. Chayote (Sechium edule) fruit shapes

Ecology and phytogeography

S. edule is grown traditionally in many regions of the world. preferably between 800 and 1800 m altitude. In many regions there are variants adapted to cultivation at sea level (in Rio de Janeiro and Yucatán); in other regions it occurs above 2000 m (in Bolivia and in the states of Oaxaca and Chihuahua in Mexico). The wild taxa closest to S. edule show a similar distribution of altitudes. since they grow between 50 and 2100 m. The chayote is cultivated in a more intensive way and for commercial purposes in Costa Rica, Guatemala. the Dominican Republic and Mexico. The range cultivated is minimal and always in accordance with consumer requirements.

The floral biology of S. edule has been studied in detail: there are various patterns in the structure and sexual expression of the staminate and pistillate flowers, which seem to be determined by genetic. environmental and seasonal factors and by the age of the plants.

Pollination is entomogamous. Among the most efficient pollinators are species of native bees of the genus Trigona, chiefly in areas of medium and low altitudes and which are free from pesticides. and the honeybee (Apis mellifera) on commercial plantations where the use of pesticides is very frequent. Secondary pollinators include wasps of the genera Polybia, Synoeca and Parachartegus.

The fruit of S. edule is viviparous, viz. the seeds germinate inside the fruit even when it is still on the plant. This characteristic does not occur in any of the wild species, in which the seeds germinate asynchronically after falling to the ground. A possible explanation for the vivipairism of the cultivate species is that the process of domestication may have resulted in suppression of the dormancy mechanisms.

Genetic diversity

Few cultivated species display the great diversity of shapes, sizes, ornamentation, armature, indumentum and colours as those found in the fruit of the S. edule. However, this diversity, which is present in the most varied combinations, has made it difficult to define cultivars. When reference is made to the different types of S. edule, therefore, it is rather in connection with local races or variants. In addition to morphological diversity, variants exist in the fruiting periods. An example of this has been observed in Oaxaca and Chiapas where local variants can yield between one and tour harvests a year. This type of variation has also been cited in the case of other regions.

The considerable diversity farmed by traditional growers contrasts with the relative homogeneity observed in fruit produced on commercial plantations. In these cases, the fruit must comply with the quality requirements demanded by the market: piriform, light green, smooth, about 15 cm long and 450 g in weight; with no physical damage or blemishes caused by pathogens; and with a suitable texture and sweet and pleasant flavour.

The wild relatives closest to S. edule are S. compositum and S. hintonii, whose distribution area is in Mexico and Guatemala. Because of a lack of agronomic evaluations, these species have not been used in genetic improvement programmes which are so necessary in the search for sources of disease resistance.

Germplasm collections. The germination characteristics of S. edule seeds do not allow them to be preserved using simple, orthodox methods. This means that the specimens have to be preserved in field collections which require careful handling.

This type of limitation is evidenced by the disappearance of some of the few collections of the genus Sechium. Between 1988 and 1990, the biggest collection of cultivated S. edule in the world (at CATIE in Turrialba, Costa Rica) as well as other smaller but equally important collections (for example at CIFAP in Celaya, Mexico) were lost. Fortunately, there are still institutions in the world that are endeavouring to preserve this important genetic stock, at least insofar as the variation of the cultivated species is concerned. Thus, in Mexico there is the collection in the hands of the UACH in Veracruz, with around 150 specimens of cultivated types from Puebla, Veracruz, Oaxaca and Chiapas. This is the only collection which currently preserves plants of some of the most important wild relatives of the chayote, such as S. compositum and the wild types of S. edule. Two other institutions caring for collections of S. edule are the Instituto Superior de Ciencias Agropecuarias of Nicaragua (Centro Experimental Campos Azules) and the Centro Nacional de Pesquisas de Hortalizas, EMBRAPA, Brazil.

Cultivation practices

S. edule is grown in the traditional way on family plots and in backyards and vegetable gardens. The viviparous characteristic of its fruit is familiar to peasant farmers, so that fruit selected for consumption is keptwithout being allowed to germinateby a small cut or puncture made in the embryos, while those selected for seed are simply allowed to ripen until it is decided to plant them.

The normal and most effective form of propagation is from seed. The most widespread sowing practice consists of planting one or more whole fruits. However, on some small holdings the seed is carefully removed and sown in pots or other media that enable it to be handled for subsequent transplantation in the final sowing plot.

In areas of traditional production, the sowing plot is prepared beforehand by making a hollow in the soil that is big enough to allow the roots to attain maximum development. Next to the sowing plots, a frame of wood or other materials is commonly erected so that the plant can grow on it quickly. For the same reasons, sowing is also frequently carried out close to a tree. During the first weeks of development, the amount of care given is relatively high (irrigation, fertilization with animal or chicken manure, etc.), although attention to the root (protection against physical damage and application of organic fertilizers) is considered of great importance throughout the plant's life cycle.

Sowing can be done at any time of the year, although it generally takes place at the beginning of the rainy season. The average length of the plants' productive cycle is three years or, in exceptional cases, eight.

On commercial plantations, sowing is carried out using rooted cuttings or selected seed. The plants are sown on permanent beds with trellises and are laid out at distances that allow the easiest possible harvesting, transport to cold-storage rooms and packaging. On the commercial type of plantations, chemical and foliar fertilizers are generally used as well as herbicides and nematicides. The leading commercial producer and exporter of chayote fruit is Costa Rica, followed by Guatemala, Mexico and the Dominican Republic.

Prospects for improvement

In spite of the fact that the whole of the S. edule plant can be used and with numerous applications (parts of the plant are used for different purposes), in several countries the majority of these uses have not become widespread and ways have not been devised to make them accessible to sectors of the population other than the peasant community.

The most widespread uses at all levels is that of the fruit as a table vegetable and in the preparation of some industrialized foods. Commercial demand requires a morphologically homogeneous production which rules out the possibility of the considerable range of fruit produced under traditional cultivation systems appearing on the market. However, as the standards required for export are very different from those accepted for the product for local consumption, it is not very likely that the usual varieties will be abandoned and that a serious genetic erosion will occur in the species. A plan to intensify and diversify S. edule production would have to include the following projects:

The establishment of permanent gene banks in several localities of Mesoamerica to maintain varietal diversity, wild populations and related congeners. These collections can be used to evaluate resistance to diseases, type of growth and organoleptic characteristics of the fruit. They will allow growers to be supplied with new sowing material and will be used as a basis for genetic improvement.

Programmes for selecting varieties with a high root yield or a high production of young stems. Both are popularly accepted consumer items, with a high nutritional value and potential use as a basic material in agroindustries.

The development of vegetative propagation methods that will provide growers with sowing material at reasonable prices.

Basic studies on the most important diseases (Ascochyta phaseolorum, Mycovellosiella cucurbiticola, Fusarium oxysporum and complexes of these and other species), particularly those which attack the fruit and which cause 35 to 40 percent of the rejects in commercial production.

Identification of problems in postharvest handling, packaging and storage during the marketing process.